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GAZA CITY (CNN) -- Jewish settlers remaining in Gaza rioted early Monday as the Israeli military began a historic withdrawal from the territory, Israeli police said.

In other parts of Gaza, police reported residents coming under fire from Palestinian militants.

There were no immediate reports of injuries from any of the incidents, which took place in the hours after Israel closed the Kissufim border crossing and 20 others that settlers used to enter Gaza.

Israel officially began its historic pullout from Gaza Sunday at the stroke of midnight (5 p.m. ET), coordinating with Palestinian forces to crack down on any violence.

Israeli officials planned to begin informing the approximately 8,500 Jewish settlers that they have 48 hours to leave Gaza or be removed by force. Some have already left.

Police reported rioting in Neve Dekalim, the largest of the settlements, around the time troops closed the crossings. At one point, about 300 people were involved, police said. But the situation was reported to be calm by 2:30 a.m. Monday.

In the southern Gaza settlement of Kfar Darom and the central settlement of Netzarim, police said Palestinian militants were shooting at settlers who remain.

But the looming possibility of violence from settlers and their ultranationalist supporters on one side, and Palestinian militant groups on the other, threatened to overshadow the optimism.

Many Gaza settlers are being forced to leave against their wishes after living for decades in the Israeli occupied territory.

Israel also is withdrawing from four Jewish settlements in the West Bank -- like Gaza, land that Israel has occupied since the 1967 Six-Day War.

Israel Defense Forces Chief of Staff Lt. Gen. Dan Halutz said Sunday the four West Bank settlements -- which are among 120 -- will be vacated after the completion of withdrawal from the Gaza settlements.

He predicted that about half the Gaza settlers will leave voluntarily. Those who don't may be removed by force as early as Wednesday.

Shortly before Sunday's midnight deadline, a steady stream of vehicles left the Gush Katif settlement block, heading toward Israel. Most had orange ribbons affixed to their cars, symbolizing peaceful disapproval of the pullouts.

Other settlers have promised to resist and have been joined in Gaza in recent days by thousands of anti-withdrawal activists. About 5,000 Israeli youths who are not settlers are inside Gaza, Halutz said.

"We're going to have to tell the government, 'No you cannot take us out of Gush Katif,' " settler Rachel Saperstein said. "We are going to stay here as long as possible, as long as our food supply holds out, our water supply and beyond that."

Israeli Defense Minister Shaoul Mofaz has predicted that all settlers will be out of Gaza within a month of disengagement.

Israel also plans to withdraw its troops from Gaza by the Jewish New Year, Rosh Hashanah, in the beginning of October.

While many settlers express religious fervor and a belief that Gaza is part of the historic Jewish homeland, some secular Israelis expressing nationalist sentiment have come to fight the pullout.

One of their chief arguments: That leaving Gaza after years of terrorist attacks by Palestinian militant groups will only reward terrorism and lead to similar tactics by groups wanting to destroy Israel.

Last week, Former Prime Minister Benjamin Netanyahu resigned as finance minister, warning Gaza will "become an Islamic terrorist base," endangering "not only Israel but many others in the world."

Meanwhile, about 20,000 Palestinian police and security forces were expected to play a role in the pullout -- 7,500 of them were deployed Sunday near Jewish settlements in southern Gaza, Palestinian security sources said.

They will focus largely on preventing attacks from militants and working to ensure that Palestinians who may move into evacuated areas are safe.

Israeli troops, larger in number and with much more training, are focusing largely on preventing violence from settlers and protecting them from militants.

Israeli government officials have said they hope the withdrawal will change the political landscape of the Middle East, opening the way for resumption of peace negotiations with the Palestinian Authority.

Palestinian security on alert

Despite complaints about a lack of equipment, Palestinian Authority security forces Saturday were on high alert, stationed outside settlements across Gaza to prevent violence in preparation for the withdrawal of Israeli troops and thousands of Jewish settlers.

Most of the settlers are to be evacuated from Gaza, where Israel has been closely coordinating with Palestinian security forces. A joint information and coordination center has been established at Erez Crossing along the northern Gaza-Israel border.

However, a senior Palestinian minister told CNN's Ben Wedeman that Palestinian security forces in Gaza have been forced to buy bullets on the black market for about $8 each because Israel's military is not providing any ammunition for the ill-equipped forces.

The atmosphere among many Palestinians is jubilant, seeing the pullout of Jewish settlers as a victory. At Gaza City's fishermen's harbor Friday, a crowd waved Palestinian flags in an official celebration of the withdrawal. Palestinian Authority President Mahmoud Abbas addressed the crowd and was given a hero's welcome.

Hamas leaders have quietly told the Palestinian Authority they will not disrupt the withdrawal process, but not everyone trusts that pledge.

In a news conference Saturday, senior Hamas member Ismail Haniyye called the withdrawal a "retreat" and said it was "a result of resistance and our people's sacrifice."

"It is evidence that resistance is able to achieve our national goals," he said.

He also vowed that the armed struggle against the Jewish state would continue, despite the pullout.

"Hamas confirms its adherence to resistance as a strategic option until the occupation retreats from our lands and holy places," senior Hamas member Ismail Haniyye said. "Our movement reaffirms that it will protect its military apparatus and Al Qassam Brigades and its weapons and keep them for defending our land."

Hamas, a Palestinian Islamic fundamentalist organization, has been labeled by the U.S. State Department as a terrorist organization. The group's military wing, Izzedine al Qassam, has admitted responsibility for terrorist attacks against Israeli civilians as well as attacks against the Israeli military.

Peres warns troops of violence

Peres met with troops Sunday at the Kissufim border crossing in Gaza, warning that they may face violence from settlers or ultranationalist supporters.

"You are saving the state of Israel," Peres said. "Israel has dangers from the outside, and [Israel Defense Forces] face them all. And now, we have a danger from the inside, and now the IDF has to deal with that as well."

"There is no sense whatsoever to remain here," he added. "The settlers do not have a future in the Gaza Strip, because they cannot live as an isolated group of people 8,000-strong among a million-and-a-half Palestinians who live in poverty and protest and unemployment."

Possible turning point

Sharon, who has invested much of his political future in the pullout's success, has said it may jump-start peace efforts between Israel and the Palestinian Authority. Although Palestinian leaders criticized Sharon for making the plan unilaterally -- rather than as part of brokered talks -- they also have expressed hope that, after 38 years in Gaza, Israel's evacuation may create a turning point.

The Bush administration supports the plan. In an interview last week with Israel's Channel 1 TV, Bush called the decision "bold."

"The disengagement is, I think, a part of making Israel more secure and peaceful, and I agree with the prime minister," Bush said. "I can understand why people think this decision is one that will create a vacuum into which terrorism will flow. I happen to disagree. I think this will create an opportunity for democracy to emerge. And democracies are peaceful."